There are conventional methods for generating and displaying a color image on a screen in a data processing apparatus. Recently, the improvement in hardware has been enhanced to enable the display of a variety of colors on the screen. However, tile conventional user interface for painting an image in desired color has not yet been known. For example, Japanese PUPA (Published Unexamined Patent Application) No.3-25687 discloses a method for producing user's desired colors from a great many colors capable of being produced and displayed by hardware. However, the method needs the preparation of an image color block (a kind of palette) occupying most of the areas on a screen, and an operation of coloring an objective image in a desired colors on the screen after the desired color was produced by using the image color block, independently of an operation of producing the desired color.